1. Field of the Invention
The present invention relates to detection of contact between a head and a disk of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.
The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is also inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.
Due to various reasons the fly height of the heads may vary during operation of the drive. Such a variation in fly height may result in poorly written data on the disk. Weakly written data may create errors during a read routine. It would be desirable to monitor the fly height of the heads. It would also be desirable to provide such a monitoring function without significantly changing the components, cost, or operation of the drive.
There have been developed heads which include a heater coil. Current is provided to the heater coil to generate heat and thermally expand the head to move the read and write elements closer to the disk. These types of heads are sometimes referred to as fly on demand (“FOD”) heads. The flying height of FOD heads can be varied by changing the amount of power provided to the heater coil.
It is desirable to create a flying height that is nearly zero. To achieve a minimum flying height it is desirable to know the point of contact between the head and the disk. Contact between the head and the disk can cause vibration and associated head movement. The vibration movement can be in a variety of directions. For example, the head can move perpendicular to and from the disk. This head movement will vary the amplitude of the read signal. There have been schemes developed to determine head contact by analyzing the amplitude of the read signal to detect mechanical vibration. The head contact may also cause cross-track vibration that varies the position error signal (“PES”) used in the servo routine of the drive. There have also been schemes to determine head contact by analyzing the PES of the servo. Head assemblies that quickly damp perpendicular or cross-track vibration limit the amount of signal data that can be analyzed to determine head contact.